CHAPTER 1
INTRODUCTION
1.1
Problem background
Water is largely taken for granted as it is perceived as the most widely
occurring substance in the Earth.
It is reported that 2.5 % of world water is
freshwater while the rest is salt. However, only 0.3 % of the world’s freshwater is
available in rivers or lake. Almost all the rest is held up by icecaps and glaciers or
buried deep in underground aquifers (Figure 1.1) (Shiklomanov, 1999).
Global freshwater consumption raised six fold between 1990 and 1995, which
is more than twice the rate of population growth. Thus, about one-third of the
world’s population already lives in countries with moderate to high water stress
(UNEP, 1999). Current predictions are that by 2050 at least one in four people is
likely to live in countries affected by chronic or recurring shortages of freshwater
(World Water Assessment Programme, WWAP, 2000).
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Figure 1.1: The water resources of earth (Shiklomanov, 1999)
Demands for water come not only from the need to drink and the need to deal
with waste. The primary consumers of water include industry as well as agriculture
sectors (Figure 1.2). Consequently, water pollution created from these demands has
significantly contributed towards the scarcity of freshwater in the world. About two
million tons of waste is dumped everyday into rivers, lakes and streams, with one
litre of wastes sufficient to pollute about eight litre of water (WWAP, 2000).
Industry
(22%)
Domestic
(8%)
Agriculture
(70%)
Figure 1.2: Global water use (UNEP, 1999)
UNEP has also stated that industrial wastes are significant sources of water
pollution. Industrial wastes often give rise to contaminant with heavy metals and
persistent organic compounds. Some 300-500 million tons of heavy metals, solvent,
toxic sludge and other wastes accumulate each year from industry (United Nations
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Industrial Development Organisation, UNIDO, 1998). Figure 1.3 shows the global
estimates of emissions of organic water pollutants by different industry sector (World
Bank, 2001). A study of 15 Japanese cities showed that 30 % of all groundwater
supplies are contaminated by chlorinated solvents from industry. In some cases, the
solvents from spills travelled as far as 10 km from the source of pollution. As a
result, strict enforcement of environmental regulations has been carried out to
minimise the water pollution.
(a)
(b)
Figure 1.3: Contributions of main industrial sectors to the production of organic
water pollutants (a) high- income countries (b) low- income countries
In most countries, industrial water tariff has been increasing from time to
time. One of the main reasons that causes this is the current inflation level, which
resulted in higher chemical cost, labour cost and construction cost. Besides, the need
for more advanced wastewater treatment techniques with higher wastewater
treatment costs to treat highly polluted water has also become one of the driving
forces towards water tariff increment. The need to fund addition of water utility to
meet rising demand for clean fresh water has also causes water supply companies to
increase the water tariff.
Therefore, rising cost of industrial freshwater and stringent environmental
regulations have been functional to reduce the water requirement from the industry.
Thus, it became necessary for the industries to look for better water management
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system to reduce their freshwater consumption and wastewater generation. To solve
this problem, many companies have applied the systematic technique based on water
pinch analysis (WPA) through efficient water utilisation.
Our experience and analysis have shown that WPA is well suited for
grassroots design but has limitations when applied to existing processes. This is
mainly caused by the existence of numerous constraints and problems related to the
operability of an existing plant. Consequently, there is a need of new systematic
techniques for retrofit of water network.
1.2
The Water Management Hierarchy
It is quite common to find the environmental issue considered during the last
stage of process design. Wastewater produced often goes through the end-of-pipe
treatment where wastewater is treated with treatment processes such as biological
treatment, filtration, membranes, etc. to a form suitable for discharged to the
environment.
Over the past decade, water minimisation through WPA has become an
important issue in the chemical process industries to achieve optimum water utility
network. This approach does achieve beneficial goals such as reducing the water
utility, bigger process throughput, lower capital and operating costs as well as
improving the public perception towards the company.
To obtain the optimum water utility design for a water network, Manan et al.,
(2004b) established a hierarchical approach for fresh water conservation called ZM
water management hierarchy (Figure 1.4). This is a general guideline for fresh water
conservation.
The hierarchy consists of five levels, namely source elimination,
source reduction, direct reuse, reclamation, and discharge after treatment. Each level
represents various water management options. The levels are arranged in order of
preference, from the most preferred option at the top of the hierarchy (level 1) to the
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least preferred at the bottom (level 5). Water minimisation is concerned with the first
to the fourth level of the hierarchy.
Source elimination and source reduction at the top of the hierarchy is
concerned with the complete avoidance of fresh water usage. When it is not possible
to eliminate or reduce fresh water at source, wastewater recycling and regeneration
should be considered. Discharge after treatment should only be considered when
wastewater cannot be recycled. Through the ZM water management hierarchy, the
end-of-pipe treatment may not be eliminated, but it will become economically
legitimate.
Source
Elimination
Source
Reduction
Reuse
Regeneration Reuse
Discharge after Treatment
Figure 1.4: A holistic approach for water minimisation through the ZM Water
Management Hierarchy (Manan et al., 2004b)
1.3
Problem Statement
Water is used in the process industry for a wide range of applications.
Increased cost of wastewater treatment and rising demand for high quality industrial
water have created a pressing need for efficient water utilisation and wastewater
reuse. The synthesis of optimal water utilisation networks has dealt with grass-root
design, where the emphasis is on the minimisation of raw water and maximisation of
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water reuse and regeneration. To date, very little has been accomplished on the use
of heuristic techniques for the retrofit of existing water network in contrast to the
work done on grassroots designs.
There is a clear need to develop systematic
techniques for water network retrofit with and without regeneration to help achieve
water savings for existing processes.
The water network retrofit problem is summarised as follows:
Given a set of mass transfer-based and/or non- mass transfer-based water-using
processes, with/without a set of treatment processes, it is desired to perform
retrofit synthesis on the existing water network with/without integration of new
treatment process(es) or optimisation of existing treatment process(es). The
various streams in the process are re-structured to simultaneously accomplish
the best savings in operating costs, subject to a minimum payback period
or/and maximum capital expenditure.
1.4
Objective
The main objective of this research is to develop new systematic techniques
for the retrofit of water network with and without regene ration that includes utility
targeting and/or network design.
1.5
Scope of Research
The scopes of this work include:
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•
Analysis of the state-of-art technique
It involved analysis of the previous approach for retrofit, their
advantages and disadvantages and the improvements required.
•
Development of retrofit targeting techniques
Three new systematic targeting techniques for water network with
and/or without regeneration have been established. These procedures
are used according to different types of water network. Capital and
operating costs as well as piping cost estimations are taken into
consideration in these targeting procedures.
•
Establishment of retrofit design procedure
A systematic retrofit design methodology has been introduced to meet
the retrofit targets. This methodology is also applicable for cases
without retrofit targeting procedure.
1.6
Research Contributions
The main contributions of this research are summarised as follows:
i. As far as it can be found in the literature, this is the first work on the
Water Cascade Analysis (WCA)-based water network retrofit synthesis.
The basic concept of pinch analysis for heat exchange network, mass
exchange network and water network are the basic of this work.
ii. A new systematic retrofit technique for water network with mass transferbased operations involving two key steps namely utility (water) targeting
and network design has been established. In the targeting stage, fresh
water and wastewater targets, and capital cost targets were determined for
a particular capital expenditure.
retrofitted to meet the targets.
Lastly the existing network was
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iii. A new systematic retrofit design methodology for non- mass transferbased operations has been established.
A new graphical tool called
concentratio n block diagram (CBD) has been introduced to diagnose,
retrofit and evolve the existing water network.
iv. A new two-stage systematic technique for the retrofit of water network
with existing regeneration unit(s) optimisation has been developed. The
first stage of the retrofit task was to locate the various retrofit targets,
where utility savings and capital investment were determined for a range
of process parameters (flowrate increment or outlet concentration
reduction of the existing regeneration unit).
Next, the existing water
network was re-designed to achieve the chosen targets.
v. A new systematic retrofit methodology, which incorporates new
regeneration unit(s) into water network retrofit has been developed. In the
targeting stage, retrofit targets (utility savings and capital investment)
were determined for a range of process parameters (total flowrate and/or
outlet concentration of the new regeneration unit) to obtain a savings
versus investment curve. Lastly the existing network was retrofitted to
meet the targets.
1.7
Summary of This Thesis
In this thesis, a set of new systematic targeting and design techniques for the
retrofit of water network have been developed.
The basic concept of pinch
technology utilised for retrofit of heat integration and mass integration has been
extended to retrofit of water network.
Chapter 2 provides a review of the relevant theories of this thesis related to
the development in pinch technology for heat exchange network, mass exchange
network and water network.
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A review of the relevant literatures of this thesis is provided in Chapter 3.
The development of pinch technology for heat exchange network, mass exchange
network and water network are reviewed.
Mathematical approaches for heat
integration are also covered in these chapters.
Chapter 4 gives an overview of the new retrofit methodologies for water
network developed in this work. Two new methods for retrofit water network are
discussed. These involve retrofit with mass transfer-based and of non- mass transferbased operations. Retrofit targeting and design procedure for water network with
mass transfer-based operations, which includes capital and operating costs
constraints are presented.
For water network with non- mass transfer-based
operations, only network design is described since no equipment investment other
than those for pipework modifications is usually required during retrofit.
The methodologies for water network retrofit with optimisation of existing
regeneration units and addition of new regeneration units are also discussed in
Chapter 4.
During retrofit targeting, various retrofit alternatives based on the
different combinations of constraints to establish the optimum retrofit targets are
examined. To achieve the targets, retrofit design is then conducted.
The detailed methodologies for retrofit of water network as well as the
analysis and discussions of the results of applying the systematic retrofit techniques
on different case studies are presented in Chapter 5.
Chapter 6 concluded the thesis by summarising the main points and
contributions discussed and exploring the potential area for future development for
water network retrofit.
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THESIS INTRODUCTION
CHAPTER 2 & 3: FUNDAMENTAL THEORY AND
LITERATURE REVIEW
A review and analysis work on:
§ Heat exchange network retrofit
§ Mass exchange network synthesis and retrofit
§ Water pinch analysis
CHAPTER 4 & 5: METHODOLOGY DEVELOPMENT AND
DISCUSSION
§ Retrofit water network with reuse and recycling
o Retrofit of water network with mass transferbased operations
o Retrofit of water network with non-mass
transfer-based operations
§ Retrofit of water network with regeneration units
optimisation
§ Retrofit of water network with the addition of new
regeneration units
CONCLUSIONS
AND FUTURE WORKS
Figure 1.5: A flow diagram illustrating the conceptual link between the chapters